Conventionally, recording electrocardiograms has been generally adopted as a technique to diagnose heart diseases.
However, conventional electrocardiography is insufficient for example to determine the location, size and geometry of a part to be treated in a heart surgery and it cannot satisfactorily locate an affected part.
This is attributed to the fact that electrocardiography is an indirect measurement methodology. Different subjects have different tissues existing between their hearts and body surfaces, different positional relationships between their hearts and other organs and bones, their respective hearts having different sizes, a different electric conductance for each tissue of their bodies, and the like. As such, it has been significantly difficult to accurately determine an affected part from information obtained from indirect measurement such as electrocardiography.
A recent study has revealed that a macular texture that is formed in a normal myocardial tissue a period of time after the onset of myocardial infarction or attributed to cardiomyopathy or other similar heart diseases induces ventricular tachycardia.
Macular texture refers to a normal myocardial tissue with a necrosed or degenerated tissue existing therein in the form of an island. In such a part of myocardium, un-uniform conduction of excitation is caused and a ventricular late potential is generated. Furthermore in such a macular texture a necrosed or degenerated tissue and a normal tissue have different electrical conduction characteristics and accordingly a double excitation conduction path (a reentry circuit) can be formed.
More specifically, an excitation signal would circle in this reentry circuit and as a result ventricular tachycardia is induced. Accordingly there exists a strong demand for three-dimensionally, accurately identifying such a part having a ventricular late potential.
Electrocardiography allows electrocardiography synchronization addition to be used to non-invasively detect whether a ventricular late potential is present or absent. However, as has been described above, it has been unable to three-dimensionally identify localization of a part in myocardium providing un-uniform conduction of excitation. An attempt has also been made to use multi-channel, electrocardiographically measured data to determine the location, size and geometry of a part having a ventricular late potential. It is, however, insufficient in precision to so determine the location and thus hardly satisfactory.
Currently, endocardial mapping using a catheter, which is a type of invasive test, is employed to observe fragmented activity to identify a part in myocardium providing un-uniform conduction of excitation. In particular, a methodology has been adapted that uses a catheter to conduct an electrophysiological test to identify a part in myocardium providing un-uniform conduction of excitation and furthermore provide a treatment by catheter with a high frequency hypersthenia (catheter ablation).
In this methodology, however, the catheter is inserted and moved while chest x-ray fluoroscopy is effected. Consequently, patients, doctors and radiographers are exposed to x-ray radiation over long periods of time. In particular, doctors and radiographers suffer large annual doses of x-ray radiation. Accordingly, there has been a strong demand for significantly reducing the time required for conducting such a test.
In a variety of fields a superconducting quantum interference device (SQUID) magnetometer has been applied. It uses an SQUID capable of detecting with high sensitivity a magnetic flux of one billionth of geomagnetism. In particular, in the field of somatometry, which strongly demands non-invasive measurement, as described above, an attempt is being made to use a SQUID magnetometer to provide a non-contact magnetic measurement of human bodies.
In particular, the development of thin-film device fabrication technology in recent years has allowed the development of a DC-SQUID, and an attempt is being made to use a SQUID magnetometer to measure a magnetocardiogram, a distribution of a magnetic field of a heart.
However, a magnetocardiogram alone cannot directly display the location, size, and geometry of a part in myocardium providing un-uniform conduction of excitation, and it hardly allows doctors to correctly understand a relative, positional relationship of an affected part in a heart.
Accordingly it has been proposed to visualize an intramyocardial, electric current behavior from a magnetocardiographic distribution represented in a magnetocardiogram. One such approach adopted is to use one or more small current element pieces (current dipoles) to mimic the source of a magnetic field for visualization. This methodology has been confirmed to be effective in determining the position of a bypass circuit (a secondary conduction path) having-peculiar electrophysiological characteristics in the WPW syndrome, e.g., a Kent bundle.
On the other hand, it has been confirmed that an excitation conduction path extending from a sinoauricular node to an atrioventricular node-a bundle of His-Purkinje fiber network can be represented by a method using the above-described current dipole to determine a source of a signal.
However, using one or more current dipoles to mimic a source of a magnetic field for visualization can only provide positional information of the current dipole(s) corresponding to a specific time point and it cannot three-dimensionally identify the location, size and geometry of a part in myocardium that has a ventricular late potential, i.e., a part in myocardium providing un-uniform conduction of excitation.
The present invention therefore contemplates an magnetocardiographic diagnosis apparatus for a ventricular late potential and a method of identifying a part in myocardium providing un-uniform conduction of excitation that can employ a non-invasive magnetic measurement to provide data representative of an intramyocardial, three-dimensional, electrical behavior and used to safely, rapidly and with high precision, three-dimensionally identify a positional relationship of a part in myocardium that has a ventricular late potential, i.e., a part in myocardium providing un-uniform conduction of excitation.